With the constantly improving quality of color photocopies and printings and in an attempt to protect security documents such as banknotes, value documents or cards, transportation tickets or cards, tax banderols, and product labels against counterfeiting, falsifying or illegal reproduction, it has been the conventional practice to incorporate various security measures in these documents. Typical examples of security measures include security threads or stripes, windows, fibers, planchettes, foils, decals, holograms, watermarks, security inks comprising optically variable pigments, magnetic or magnetizable thin film interference pigments, interference-coated particles, thermochromic pigments, photochromic pigments, luminescent, infrared-absorbing, ultraviolet-absorbing or magnetic compounds.
Security threads embedded in the substrate are known to those skilled in the art as an efficient measure for the protection of security documents and banknotes against imitation. Reference is made to U.S. Pat. No. 0,964,014; U.S. Pat. No. 4,652,015; U.S. Pat. No. 5,068,008; U.S. Pat. No. 5,324,079; WO 90/08367; WO 92/11142; WO 96/04143; WO 96/39685; WO 98/19866; EP-A 0 021 350; EP-A 0 185 396; EP-A 0 303 725; EP-A 0 319 157; EP-A 0 518 740; EP-A 0 608 078; EP-A 0 635 431; and EP-A 1 498 545 as well as the references cited therein. A security thread is a metal- or plastic-filament, which is incorporated during the manufacturing process into the substrate serving for printing security documents or banknotes. Security threads or stripes carry particular security elements, serving for the public- and/or machine-authentication of the security document, in particular for banknotes. Suitable security elements for such purpose include without limitation metallizations, optically variable compounds, luminescent compounds, micro-texts and magnetic features.
With the aim of protecting value documents such as banknotes from being forged, optically variable security threads or stripe exhibiting color shift or color change upon variation of the angle of observation have been proposed as security features to be incorporated into or onto said value documents. The protection from forgery is based on the variable color effect that optically variable security elements convey to the viewer in dependence on the viewing angle or direction.
WO 2004/048120 discloses security elements comprising at least two adjacent regions, wherein one of the regions is an optically variable and the other region has a layer of material with constant reflection. The disclosed security element comprises regions forming areas without material in order to form graphic makings, characters and the like that can be detected visually.
US 2007/0241553 discloses security elements for securing valuable articles having an optically variable layer that imparts different color impressions at different viewing angles and, in a covering area, a semi-transparent ink layer disposed on top of the optically variable, the color impression of the optically variable layer being coordinated with the color impression of the semi-transparent ink layer in the covering area when viewed under predefined viewing conditions.
WO 2007/042865 discloses security elements comprising at least two contiguous areas having an identical or different optically variable coloring. The disclosed security element further comprises a single graphic marking which crosses with continuity the two areas having variable coloring so that the graphic marking straddles the two areas and is perfectly aligned.
US 2011/0095518 discloses security elements for securing valuable articles comprising a stack layer made of an optically variable layer that conveys different color impressions at different viewing angles, and a color-constant layer comprising an ink layer and a metal layer. The optically variable layer and the color-constant layer are stacked in a covering region, while at most one of the optically variable layer and the color-constant layer is present outside the covering region. The color impression of the stacked layers in the covering region and the color impression of the one layer outside the covering region are matched with each other when viewed at a predetermined viewing angle.
EP-A 2 465 701 discloses security elements for securing valuable articles comprising a stack layer made of an optically variable layer that conveys different color impressions at different viewing angles, a first portion with a first color-constant impression and a second color-constant impression and an individualizing marking. The optically variable layer and the two portions exhibiting two color-constant impressions are stacked in a covering region. The disclosed different layers are coordinated so that the color impression of the optically variable layer matches at a predetermined first viewing angle the color impression of the first portion and that the color impression of the optically variable layer matches at a predetermined second viewing angle being different from the first viewing angle the color impression of the second portion.
WO 2011/107527 discloses threads or stripes comprising a hardened coating comprising oriented magnetic or magnetizable pigment particles, in particular optically variable magnetic or magnetizable pigments particles, said orientation of pigment particles representing graphic information.
A need remains for providing more sophisticated security threads or stripes so as to further increase the resistance against counterfeiting or illegal reproduction of security documents comprising said security threads or stripes.